Unit of Heat – SI Unit

Last Updated : 14 Mar, 2024

Unit of Heat is Joule. It is named after the scientist James Prescott Joule in recognition of his contributions to research in thermal sciences. Heat is a form of energy which deals with the body’s temperature. Heat energy flows from one body to another due to their temperature difference. Heat is also measured in calories or kilocalories.

In this article, we will learn the definition of heat, its unit in various systems, conversion between different heat units, and some practice problems based on it.

Definition of Heat

Heat can be defined as follows,

Heat is the thermal energy which is transferred between two systems due to temperature difference in them.

The symbol ‘Q’ in physics and thermodynamics denotes heat.

Formula of Heat

The formula to calculate heat is given as follows:

Q = mc Î”T

where,

Q= heat,

m = mass of the body,

c = specific heat, and

Î”T = temperature difference.

What is Unit of Heat?

Unit of heat is a measure of the amount of thermal energy transferred between two substances due to a temperature difference. It quantifies the amount of heat energy involved in a process. Thus, it is a standard amount of heat chosen for measurement in relation to this quantity.

As heat is calculated by the formula Q = mc Î”T. Hence, one unit of heat can also be defined as amount of heat transferred through a body having mass of 1 kg, specific heat of 1 J/kg-K and a temperature difference of 1 Kelvin.

For convenience, various units of heat have been adopted. The SI unit of Heat is Joule (J).

Dimensional Formula for Heat

The dimensional formula for heat is [M1 L2 T-2]. It is derived using the analogy of heat to energy. Energy is equivalent to the work done which is defined as product of Force and displacement, i.e. Work done = Force Ã— Displacement.

Substituting the dimensions of Force and Displacement, we get,

Dimensions of Work Done = [M1L1T-2] Ã— [M0L1T0] = [M1 L2 T-2]

Dimensional Formula of Heat = [M1 L2 T-2]

SI Unit of Heat

S.I. Unit of Heat is Joule (J). It is named after the scientist James Prescout Joule. The base unit of heat is same as that of energy and work, i.e. kgms-2.

What is 1 Joule?

1 Joule is defined as amount of work done or energy stored in a body if a force of 1 Newton displaces it through 1 meter.

Unit of Heat in CGS System

C.G.S. system of units stands for Centimeters (length), Grams (mass), and Seconds (time). In CGS system, the unit of heat is Calorie abbreviated as Cal.

1 Calorie of heat is defined as amount of heat required to raise temperature of 1 gram water by 1 degree celsius. It is found to be approximately equal to 4.2 Joules.

For convenient measurements, the unit kilocalorie (kcal) is used sometimes which is equal to 1000 calories.

Unit of Heat in Other Systems

Unit is a standard measurement of any physical quantity that is used to express the size or magnitude of any physical quantity. There are different units chosen for the measurement of same physical quantity for its convenient representation in terms of magnitude. The unit of heat in different systems is mentioned below:

• MKS System: Joule
• CGS System: Calorie
• Imperial System: BTU (British Thermal Unit)

Similar to other SI units, the value of Joule can be adjusted to different measures as shown in the table below:

Unit of Heat

Symbol

Equivalent in Joule (J)

Joule

J

1

Millijoule

mJ

0.001

Microjoule

Î¼J

0.000001

Nanojoule

nJ

0.000000001

Kilojoule

kJ

1000

Megajoule

MJ

1000000

Gigajoule

GJ

1000000000

Conversion Between Unit of Heat

Unit of heat can be converted from one system to another using the conversion factors given in the following table:

From/To

Joule

Calorie

Kilocalorie

BTU

Joule

1

0.239006

0.000239006

0.0009478

Calorie

4.184

1

0.001

0.00396567

Kilocalorie

4184

1000

1

3.96567

BTU

1055.06

252.164

0.252164

1

Using the above conversion system we can convert unit of heat from one form to other form.

Difference Between Heat and Temperature

The difference between heat and temperature is described in tabular form as follows:

Heat

Temperature

Heat is a form of energy that flows from one system to another due to temperature difference between them.

Temperature is a measure of hotness or coldness of a body or system.

SI unit of heat is Joule.

SI unit of Temperature is Kelvin.

Other units of heat include calorie, BTU, etc.

Other units of temperature are Celsius, Fahrenheit, etc.

Solved Examples on Unit of Heat

Example 1: Calculate the amount of heat required to raise temperature of 100 grams of water by 20 degree Kelvin. Given that, specific heat of water = 4.2 J/kg-K.

Solution:

We have, Mass of water, m = 100 grams = 0.1 kg

Specific Heat of water, c = 4.2 J/kg-K

Temperature difference, Î”T = 20 K

Also, Heat required, Q = mcÎ”T

Thus, Q = 0.1 Ã— 4.2 Ã— 20 = 8.4 J

Hence, 8.4 Joules of heat is required for the given case.

Example 2: Convert 120 kcal into SI units.

Solution:

We know that, 1 cal = 4.184 J and 1 kcal = 1000 cal

Thus, 120 kcal = 120 Ã— 1000 cal = 120000 Ã— 4.184 J = 502080 Joules

Hence, the given amount of heat is 502080 J in SI units.

Practice Problems on Heat Units

Problem 1: Convert 100 J of heat into calories.

Problem 2: Find the amount of heat required to raise temperature of 200 grams of water by 10 degree Kelvin.

Problem 3: Convert the heat given in SI units as 70 J into CGS units.

Problem 4: 1 kg of water is at 25 degree celsius. What is the amount of heat required to raise temperature of this water to 100 degree celsius?

Problem 5: Convert 1 kcal of heat into SI units.

Unit of Heat FAQs

What is SI unit of heat?

The SI unit of heat is Joule (J).

What is unit of heat in CGS system?

The unit of heat in CGS system is Calorie (cal).

What is meant by specific heat capacity?

Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius or Kelvin.

Are heat and temperature same quantities?

No, heat and temperature are not the same. They are related concepts, but they refer to different physical quantities. As heat is the transfer of energy due to temperature difference, while temperature is a measure of the average kinetic energy of particles in a substance.

What is the unit of specific heat capacity?

Unit of specific heat capacity is generally taken as J/kg-K.

What is meant by latent heat?

Latent heat is the amount of heat energy absorbed or released by a substance during a phase change, i.e. melting, vaporization, etc. without a change in temperature.